1,721,005 research outputs found
Splay-bend surface elastic constant of nematic liquid crystals: A solution of the Somoza-Tarazona paradox
No full textThe Nehring-Saupe [J. Chem. Phys. 54, 337 (1971); 56, 5527 (1972)] elastic free energy of nematic liquid crystals (NLCs) contains the splay-bend elastic constant K-13, which affects only the elastic surface free energy. Several years ago, Somoza and Tarazona [Mol. Phys. 72, 991 (1991)]. showed that the value of K-13 depends on the nonlocal to local mapping that is used to define the local elastic free energy. Then they concluded that the splay-bend constant is not a well-defined physical parameter. In the present paper we show that the Somoza-Tarazona result comes from an inconsistent treatment of the boundary effects. If all the boundary effects are correctly taken into account in an elastic approach, the elastic surface free energy contains an effective elastic constant K-13(eff) that is mapping independent. K-13(eff) is the sum of three different constants: the classical Nehring-Saupe bulk constant K-13 and two specific interfacial constants K-1 and K-h. While each surface constant (K-13, K-1, and K-h) depends on the kind of nonlocal to local mapping, the resulting surface constant K-13(eff)=K-13+K-1+K-h is mapping independent. Using a simple molecular model of the intermolecular interactions, we obtain explicit expressions of K-13(eff) in terms of the characteristic parameters of the intermolecular energy. In the final part of this paper we discuss the meaning and the physical consequences of the elastic surface free energy F-s. We show that F-s is a semimacroscopic; parameter that provides an approximate elastic description of the interfacial layer. Furthermore, we point out that the elastic surface free energy should not be confused with the thermodynamic surface free energy that appears in a consistent continuum theory of NLCs
Heterogeneities at short and long time scales of the dynamics of a probe molecule in different model glass formers
No full textThe correlations, namely dynamic heterogeneity, and different relaxation time and length scales were investigated in two model glass formers. Decoupling phenomena were observed on the transport coefficients, which would correspond to different averages over these heterogeneities. The temperature dependence of the rotational dynamics of a nearly cylindrical molecular tracer in phenyl salicylate (SALOL) was investigated over the short time scale of the Longitudinal Detected Electron Spin Resonance spectroscopy (LODESR). A comparison with the dynamics of the same probe dissolved in either o-terphenyl (OTP) or SALOL is also reported on the short and long time scales of the rotational relaxation, the latter being provided by linear Electron Spin Resonance (ESR) studies. The rotational dynamics, at short and long times, exhibited a similar behavior in both glass formers with exponential relaxation and coupling to the viscous flow at higher temperatures, and with selective decouplings on cooling. Interestingly, by proper scaling, the dynamics at long times and the viscosity of the host matrices collapse in a single curve in a wide temperature range. The scaling does not work at short times. (c) 2006 Elsevier B.V. All rights reserved
On the scaling in the rotational dynamics of molecular probes in salol and ortho-terphenyl: a possible role of the eneirgy landscape basins
No full textThe reorientational dynamics of a stiff paramagnetic tracer dissolved in the glass former salol is investigated by means of electron spin resonance over a broad temperature range. The Debye-Stokes-Einstein law describing rotational diffusion in simple liquids, is found to break down in the supercooled region where the diffusion is less temperature dependent than the viscosity. Over a large temperature interval a simple power law relates diffusion and viscosity, whereas at lower temperatures the decoupling is stronger and an activated dynamics is observed. These experiments are discussed together with previous data concerning other tracer/glass former couples. Starting from some observed common features, an attempt is made to obtain a unifying interpretation of the data in the framework of the energy landscape picture
Fractional Debye Stokes Einstein law and scaling of the rotational relaxation in molecular glass formers: linear and non-linear ESR studies
No full textThe Stokes Einstein and Debye Stokes Einstein laws, which relate viscosity and diffusion, hold in liquids. However, deviations are observed in supercooled liquids, which lead to decoupling of the translational motion from the viscosity. This new dynamical regime is well accounted for by assuming a power relation between viscosity and diffusion.
In this work, evidences for the presence of power law relating viscosity and rotational relaxation (Fractional Debye Stokes Einstein law) obtained with linear and non linear ESR experiments will be reviewed. The rotational dynamics at long ad short times of nearly spherical and cylindrical tracers in supercooled and glassy molecular glass formers will be considered. At higher temperatures exponential relaxations are found. Decouplings of the rotation and FDSE behaviours are observed on cooling both on long and short times scales of the relaxation and for both the different length scales probed by the different tracers. Suitable scaling procedures will be discussed to collapse data in a master curve
Scaling analysis and distribution of the rotational correlation times of a tracer in rubbery and glassy poly(vinyl acetate): An electron spin resonance investigation
No full textWe studied by ESR the rotational motion of a paramagnetic tracer (a deuterated N-15-enriched nitroxide) in poly(vinyl acetate) (PVAc). The reorientation of the tracer occurs via jumps of about 50 degrees with a heterogeneous distribution of correlation times. Depending on the temperature range, the average correlation time [tau] scales as the alpha, beta, and gamma relaxation times of PVAc, i.e., [tau] = C-i[tau](i), i = alpha, beta, gamma. On cooling, [tau] tracks the alpha relaxation to about the alpha-beta bifurcation temperature and then the beta relaxation down to the glass transition. In glassy PVAc [tau]/[tau] = C-gamma approximate to 1
Jump rotation in a molecular glass former
No full textThe rotational dynamics of the spin probe 2,2,6,6-tetramethyl-piperidin-loxyl (TEMPO) dissolved in the molecular glass former phenyl salicylate (Salol) have been investigated via electron spin resonance spectroscopy. in a temperature interval ranging from the supercooled to the glassy state of Salol. The comparison of experimental data with predictions of both difusive and jump reorientational models has been made. It has been proved that in both the supercooled and the glassy state, the TEMPO spin probe rotates according to a jump process. In the investigated temperature range, the angular jump size Phi has been found to be constant and holds 75degrees. Moreover, the temperature dependence of the reorientational correlation times. which is well described by a double Arrhenius law. has been studied, The crossing temperature from the first to the second Arrhenius regime is found to be near to the glass transition temperature. Comparison with the results of a previous study on the reorientational dynamics of a TEMPO probe in o-terphenyl, has shown that rotational motion happens in structural cages with comparable free volumes
Scaling of the rotational relaxation of tracers in o-terphenyl: A linear and nonlinear ESR study
No full textThe rotational dynamics at long and short times of nearly spherical and cylindrical tracers in supercooled and glassy o-terphenyl is investigated by linear and nonlinear electron spin resonance spectroscopy, respectively. The short-time and long-time dynamics are characterized by the effective correlation times tau(s) and tau(l), respectively. At higher temperatures, the relaxation is exponential and the Debye-Stokes-Einstein law (DSE) holds, tau(s) approximate to tau(l) proportional to eta. eta is the shear viscosity. On cooling, the rotational dynamics is partially coupled to the viscosity. At lower temperatures the reorientation is activated. Interestingly, on the short time scale the rotational correlation times of the two tracers (tau(s)) in a wide temperature range down to the activated regime collapse in a single curve by proper scaling. The scaling does not work at long times (tau(l))
Heterogeneity in the dynamics of a molecular tracer dissolved in liquid crystal homopolymer and copolymer
No full textESR studies were carried out on the rotational dynamics of cholestane molecular tracer dissolved in liquid crystal homopolymer and copolymer from well above T-NI down to below T-g Rheological measurements also provided the flow relaxation properties of the polymer matrices. The distribution functions of molecular sites were determined, and their stability was monitored as a function of thermal history. The different molecular site distributions and cooperativity degrees were compared and discussed
Enthalpy relaxation of low molecular weight PMMA: a strategy to evaluate the Tool-Narayanaswamy-Moynihan model parameters
No full textThe enthalpy recovery mechanism of a low molecular weight synthesis of polymethylmethacrylate is investigated by means of differential scanning calorimetry (DSC) experiments. The experimental results can be described satisfactorily in terms of the Tool-Narayanaswamy-Moynihan theory. This work is mainly focused on developing a strategy for evaluation of the best set of parameters for the model. The approach starts with a simultaneous fitting procedure of several experimental DSC traces. Sets of parameters are obtained which exhibit agreement with experiments. The enthalpy lost on ageing of the sample in the glassy state as a function of the annealing time is then compared with the predictions provided by using the different sets of parameters. We show that this procedure is able to single out the best set of parameters and to provide a good estimation of the Adam-Gibbs temperature
Enthalpy recovery in low molecular weight PMMA
No full textThe phenomenological model of Tool-Narayanaswamy-Moynihan (TNM), has been widely used in order to describe enthalpy and volume relaxation experiments in different glass forming systems. However some difficulties were evidenced, especially in polymeric samples. In this work we study the enthalpy relaxation mechanism of a low molecular weight poly(methyl methacrylate) (PMMA) sample via differential scanning calorimetry experiments. We show that the TNM model, with the Scherer-Hodge expression for relaxation times, describes the aging mechanism fairly well so that it was possible to simultaneously reproduce different experimental scans with a single set of parameters. Furthermore the long term enthalpy lost after annealing was found to agree with the prediction of the model. On the contrary, from different papers in the literature it is found that the TNM model shows severe difficulties in describing the enthalpy recovery mechanism of high molecular weight PMMA samples. These results could support the idea that chain entanglements play a role in the difficulties encountered by TNM model in high polymers as suggested by others in the past. (C) 2003 Elsevier B.V. All rights reserved
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